This invention pertains to electrostatic lenses and electrostatic optical systems as used in charged-particle-beam (CPB) 2-D mapping microscopes (e.g., mapping electron microscopes) and related systems for observing a surface of a specimen in two dimensions.
A scanning electron microscope (SEM) generally is used for examining the surface of a specimen, such as the product of a step in a process for manufacturing semiconductor integrated circuits, especially to ascertain the presence of surficial defects. In view of the fact that an electron beam is an exemplary charged particle beam, work has been done directed to the use of other charged particle beams (such as a focused ion beam) for similar applications.
Since principles generally applicable to an electron beam are applicable to an ion beam, the discussion below is made in the context of an electron-beam system. However, in view of the above, it will be understood that the invention is not limited to electron-beam systems.
In an SEM, as is generally known, an electron beam is irradiated onto a point on the surface of the specimen being observed. Impingement of the electron beam on the specimen surface causes the surface to emit secondary electrons. The secondary electrons are accelerated away from the surface, collected, and quantified by a suitable detector. To image a region on the sample, the electron beam simply is scanned in two dimensions in a raster manner. Secondary electrons generated at each irradiation point in the scan are collected and quantified. The data collected by the detector are processed to form an image that is displayed on a screen (CRT) or the like.
A main disadvantage of conventional SEMs is the long period of time required for obtaining an image of the surface being observed. The time is related to the need to two-dimensionally scan a point-focused electron beam over the observed surface. As a result, xe2x80x9cmapping electron microscopesxe2x80x9d are being investigated for use, as a possible alternative to SEMs, in examining semiconductor wafers and chips and in other applications in which high speed is required. This is because a mapping electron microscope offers prospects of simultaneously viewing an entire region of the target surface in two dimensions. To such end, a mapping electron microscope utilizes an electron-optical system (i.e., a system comprising multiple electron lenses) to direct the electron beam onto an area of the sample surface that is larger than a point. Unfortunately, various technical problems remain unresolved with mapping electron microscopes.
One problem concerns the substantial aberration that is encountered whenever a wide visual field is imposed on an electron-optical system. A conventional electron-optical system for use in a mapping electron microscope utilizes multiple electrostatic lenses to achieve a suitably high magnification of the image of the target surface. In such systems, simple Einzel (unipotential) lenses typically are used for all lenses except for the initial (cathode) lens. However, with such lenses, suitably large fields cannot be obtained because large fields produce serious aberrations.
With simple Einzel lenses, image-degrading aberrations can be reduced somewhat by forming the image using two lenses. However, such a configuration cannot produce the desired high image magnification.
More specifically, whenever high-magnification enlargement and projection are performed using electrostatic lenses, a simple short-focal-length (f) lens may be situated an axial distance (f+dz), wherein dz less than  less than f, to the rear of the object plane or of an intermediate imaging plane. However, if the voltage applied to the electrostatic lens is increased, the field intensity within the lens increases and f shortens. If the impressed voltage is excessive, a potential barrier is formed that exceeds the kinetic energy of the electrons in the beam. In such a condition, the electrons are repelled by the lens and the desired lensing action is not obtained.
At a given impressed voltage to an electrostatic lens, the focal length f can be shortened simply by making the lens smaller. However, this approach is problematic in that there are practical limitations on the spacing between adjacent electrodes of the lens. These limitations mainly concern, for example, breakdown voltages between the electrodes. Also, off-axis aberrations tend to be excessive whenever small electrostatic lenses are used that have narrow on-axis fields. Therefore, it has been difficult to construct short-f electrostatic lenses that exhibit acceptably reduced aberrations.
Conventional approaches that achieve increased magnification by multi-stage imaging, especially in systems comprising multiple axially aligned simple Einzel lenses, have other problems. For example, whenever an electron beam passes through a lens located remotely downstream, portions of the beam pass through extreme off-axis regions of the lens. In such situations, even though aberrations could be suppressed adequately by making the remote lens extremely large, this approach is impractical. Alternatively, the electron-optical system is made extremely long in the axial direction so as to achieve high magnification with adequate suppression of aberrations. The great length of such a system is a serious disadvantage.
In other types of conventional electron-beam mapping-projection apparatus, as shown in FIG. 9, an xe2x80x9cE cross Bxe2x80x9d (xe2x80x9cExBxe2x80x9d; sometimes also termed a Wien filter) 42 is used to achieve perpendicular irradiation of the target surface 44 with an electron beam passing through an xe2x80x9cirradiation columnxe2x80x9d 41. Secondary electrons emitted from the target surface 44 are routed through a xe2x80x9cprojection systemxe2x80x9d PL having an optical axis AX that is perpendicular to the target surface 44. More specifically, the xe2x80x9cirradiation beamxe2x80x9d (having a predetermined transverse area) approaches the ExB 42 along an axis Al that is angled relative to an optical axis AX. The irradiation beam is directed to the optical center of the ExB 42. Upon passing through the ExB 42, the irradiation beam propagates along the optical axis AX through a front lens 43 to the target surface 44 to xe2x80x9cdown-illuminatexe2x80x9d (irradiate) the target surface at a zero angle of incidence. Secondary electrons emitted by the target surface 44 return along the optical axis AX through the front lens 43 and pass straight through the ExB 42 without being deflected. Upon passing through the front lens 43, the secondary electrons form a first intermediate image at a first intermediate-imaging plane Ml situated at the optical center of the ExB 42. An aperture 45 is provided to decrease aberrations in the first intermediate image.
From the ExB 42, the beam of secondary electrons enters the projection system PL. The projection system PL comprises first and second projection lenses 46, 47, respectively. An image of the first intermediate image is formed, as a second intermediate image, at a second intermediate-imaging plane M2 by the first projection lens 46. An image of the second intermediate image is formed on a detector surface (imaging surface) 48 by the second projection lens 47. The overall magnification of the image on the detector surface 48 can be varied in a continuous manner (i.e., xe2x80x9czoomedxe2x80x9d) by varying the electrical energy supplied to the first projection lens 46, which varies the position of the second intermediate-imaging plane M2.
The first intermediate-imaging plane M1 is located at the optical center of the ExB 42 to eliminate, in a substantial manner, chromatic aberrations of the ExB 42 arising from its function as a conventional Wien filter, and to eliminate, in a substantial manner, the astigmatism that is characteristic of ExBs.
Unfortunately, a secondary-electron mapping-projection system such as that shown in FIG. 9 has key disadvantages. First, the axis distance between the ExB 42 and any lenses downstream of it must be very large. As noted above, aberrations caused by the ExB 42 can be reduced by situating the first intermediate imaging plane M1 at or near the optical center of the ExB. However, for example, when enlargement projection is performed using a single projection lens downstream of the ExB 42, since the distance between the lens and the first intermediate imaging plane M1 is large, the overall axial length required to form the image is extremely long. Also, image xe2x80x9czoomingxe2x80x9d (continuous change in magnification) cannot be performed because the first intermediate imaging plane M1 is fixed.
The conventional manner of solving this problem is to insert multiple projection lenses between the imaging plane M1 and the surface of the detector 48, as shown in FIG. 9. This allows the second intermediate imaging plane M2 to be shifted (xe2x80x9czoomxe2x80x9d arrow) within a limited range along the axis AX to achieve certain high magnifications. However, off-axis aberrations are increased unacceptably when attempting to use such a configuration to view the target surface in a wide-field, low-magnification range. I.e., under the latter conditions, a principal ray P (solid line) propagates divergently through the first intermediate image plane M1 and thus enters the first projection lens 46 far off-axis (due to the long axial distance between the ExB 42 and the first projection lens 46). This situation causes an unacceptable increase in off-axis aberrations such as pincushion distortion. Also, whenever an image at the first intermediate imaging plane M1 is enlarged by the projection lenses 46, 47 and formed on the surface of the detector 48, the principal ray P incident divergently off-axis to the second projection lens 47 passes through that lens, thereby increasing pincushion distortion.
In view of the shortcomings of conventional mapping-projection-optical systems, as summarized above, an object of this invention is to provide electrostatic lens systems and mapping-projection systems comprising such lenses that achieve high magnification with excellent control of aberrations, even when the length of the optical system is relatively short and the optical field is wide.
Another object is to provide, for secondary electrons, mapping-projection-optical systems including a beam separator (e.g., an ExB), in which systems distortions are controlled over a wide optical field and that provide a zooming range from low to high magnification while maintaining excellent imaging quality.
To such ends and according to a first aspect of the invention, electrostatic projection-lens systems are provided for secondary-electron mapping-projection apparatus. One embodiment of such a system comprises at least five electrodes arranged along an optical axis. The at least five electrodes comprise a middle electrode, a front-power group disposed axially upstream of the middle electrode and including at least one front electrode, and a rear-power group disposed axially downstream of the middle electrode and including at least one rear electrode. Each power group includes at least one electrode that functions as a respective lens so as to contribute a front power and rear power, respectively, to an overall power of the projection-lens system. Each of the front and rear powers is independently variable.
In such a configuration in which the front and rear powers are independently variable, e.g., the electrodes of the rear power can be used as an imaging lens and the electrodes of the front power can be used for controlling the principal ray so that the ray trajectory extends paraxially (along the optical axis near the center of the deflectors of the rear power). With such a configuration, distortion aberrations are minimized even in a wide optical field.
No distortion is produced in an optical system in which the principal ray intersects the axis at the front nodal point of the system (the front nodal point is the same as the principal point of the system if the system is surrounded on both sides by a medium having the same refractive index). Barrel distortion is produced if the principal ray intersects the axis closer to the object plane than the front nodal point, and pincushion distortion is produced if the principal ray intersects the axis closer to the image plane of the system than the front nodal point. Distortion aberrations can be reduced if a field lens is placed at an intermediate imaging position. The configuration of the principal ray incident to the optical system is controlled by adjusting the power of the field lens.
Upon applying these principles to an electrostatic imaging lens, under conditions in which distortions are not produced, the principal ray at any particular field angle intersects the axis near the center (principal point or nodal point) of the imaging lens (principal point=nodal point). Aberrations can be well suppressed even if an electrostatic imaging lens is used that is smaller and has a narrower paraxial field than usual. Hence, small electrostatic lenses can be used even when higher-magnification enlargement projection is desired, allowing focal lengths to be reduced further. The incidence of the principal ray to the imaging lens is controlled using a field lens (corresponding to the front power). With such a configuration, aberrations are minimized and high magnification is obtained even when using a small electrostatic imaging lens to image a wide optical field.
Desirably, the potential of the front-most electrode (in the front-power group) and the potential of the rear-most electrode (in the rear power group) are at ground potential. The middle electrode is optionally, not necessarily, at ground potential. If the respective potentials of the front-most and rear-most electrodes and the potential of the middle electrode all are equal, then a configuration and lens action are achieved that are the same as if two Einzel lenses were combined. If the front-most, rear-most, and middle electrodes are not all at ground potential, then a bi-potential lens assembly can be realized by varying the respective potentials supplied to these electrodes. Hence, it is possible, for example, to place the center of the front power and the center of the rear power closer to the middle electrode than their respective actual electrode positions.
In the projection-lens system summarized above, at least one of the front and rear power groups can include, in combination with the middle electrode, at least four electrodes. In such a configuration, each of the front and rear powers has a respective center position. The respective center position of at least one of the front and rear powers is varied by varying a respective voltage applied to the respective electrode.
In a configuration in which the front power group includes at least four electrodes, it is possible to vary the front power and its center position independently by adjusting the voltages impressed on at least two electrodes of the group that function as respective lenses. By adjusting the center of the front power so as to place this center at an intermediate image plane formed by a lens system located axially upstream of the subject projection-lens system, the principal ray can be made to intersect the optical axis near the center of the rear power simply by adjusting the front power. If (1) the position of the intermediate image plane is changed by the upstream lens system, (2) the respective voltages impressed on the two (or more) electrodes are adjusted so that the center of the front power remains at the intermediate image plane and the principal ray intersects the optical axis near the center of the rear power, then image magnification can be varied continuously (xe2x80x9czoomedxe2x80x9d) over a desired range.
In a configuration in which the rear power includes at least four electrodes, it is possible to vary the rear power and its center position independently by adjusting respective voltages impressed on at least two electrodes serving as respective lenses. Varying the position of the center of the rear power is equivalent to varying the position of an imaging lens, whereby the image magnification can be varied continuously. Also, by varying the center position of the rear power in addition to varying the center position of the front power, the principal ray can be controlled so that it always intersects the optical axis in the vicinity of the center of the rear power. This configuration provides great flexibility in changing image magnification while controlling aberrations.
At least one electrode in the front-power group can be configured as a multi-pole electrode including at least four poles. Such a configuration is especially useful for reducing astigmatism whenever a beam separator (e.g., an ExB) is used to convey down-lighting irradiation to the specimen surface. I.e., in a configuration including a beam separator, the angle of the principal ray at the intermediate imaging plane can be different depending upon the orientation of the principal ray in the transverse (x-y) plane orthogonal to the optical axis. Under such conditions, a quadrupole facilitates placement of the principal ray so as to intersect the optical axis at or near the center of the rear power in all directions of the transverse plane. To such end, the quadrupole can vary the deflection angle, within the transverse plane, of the principal ray as achieved by the front power.
It is desirable, in an electrostatic projection-lens system as summarized above, that the center of the front power be situated at or nearly at the intermediate imaging plane. Normally, in order for the principal ray to intersect the optical axis near the center of the rear power, it generally is necessary to adjust the upstream lens system at the same time as the front power is adjusted. However, if the center of the front power is situated at or nearly at the intermediate imaging plane, then the principal ray can be made to intersect the optical axis near the center of the rear power simply by adjusting the front power, thereby greatly simplifying adjustment.
The phrase xe2x80x9cat or nearly atxe2x80x9d the intermediate imaging plane means that it is not necessary that the center of the front power be situated exactly at the intermediate imaging plane. The center can positioned, within a predetermined tolerance, at the intermediate imaging plane, wherein the tolerance is dictated by design parameters such as permissible aberration level, etc. The magnitude of the tolerance can be determined readily by persons of ordinary skill in the relevant art.
According to another aspect of the invention, secondary-electron mapping-projection apparatus are provided that comprise a projection-lens system as summarized above. A mapping-projection apparatus according to this aspect provides good aberration control at high magnification and with a large optical field with a relatively small axial configuration.
According to another aspect of the invention, methods are provided for performing secondary-electron mapping-projection microscopy, in which an image of a specimen surface is formed at a pre-determined magnification. In one embodiment of such a method, an electrostatic projection-lens system is provided that comprises at least five electrodes arranged along an optical axis. The at least five electrodes comprise a middle electrode, a front-power group disposed axially upstream of the middle electrode and including at least one front electrode, and a rear-power group disposed axially downstream of the middle electrode and including at least one rear electrode. Each power group includes at least one electrode contributing a front power and rear power, respectively, to an overall power of the projection-lens system, and each of the front and rear powers is independently variable. A region of the specimen surface is irradiated with a charged particle beam so as to cause the specimen surface to emit secondary electrons. The secondary electrons are routed into an end of the projection-lens system. At least one of the front power and rear power is adjusted to obtain an electron-image of the irradiated region of the specimen surface at a desired magnification.
According to yet another aspect of the invention, secondary-electron mapping-projection apparatus are provided. One embodiment of such an apparatus comprises, along an optical axis perpendicular to a specimen surface, a beam separator (e.g., an ExB), a front lens system, a projection-lens system, and an imaging surface. The beam separator is situated and configured to cause an irradiation charged particle beam, incident to the beam separator from an off-axis source, to propagate along an optical axis to a specimen surface such that the irradiation beam is incident perpendicularly to the specimen surface. The irradiation beam has an energy sufficient to cause the specimen surface to emit secondary electrons. The front lens system is situated along an axis between the beam separator and the specimen surface. The front lens system is configured bi-directionally telecentrically and is configured to form the secondary electrons into a beam that propagates along the axis from the specimen surface and forms an intermediate image of the irradiated surface at an intermediate image plane at an optical center of the beam separator. The projection-lens system is situated axially between the beam separator and the imaging surface. The projection-lens system is configured to cause the beam of secondary electrons to propagate along the axis and form an image of the intermediate image on the imaging surface.
By providing the front lens system with a substantially bi-directionally telecentric configuration, a principal ray will be incident to a first lens of the projection-lens system without any divergence. Thus, aberrations (e.g., distortions) can be reduced even when performing wide-field imaging. In addition, the bi-directionally telecentric aspect of the front lens system prevents peripheral light dampening of the specimen surface, thereby allowing viewing with uniform brightness.
The phrase xe2x80x9csubstantially bi-directionally telecentricxe2x80x9d means that exact bidirectional telecentricity is not required. Actual telecentricity can vary slightly from exactness so long as actual aberrations are maintained within allowable ranges. The allowable ranges are determined easily by a person of ordinary skill in the relevant art.
Another embodiment of a secondary-electron mapping-projection apparatus according to the invention comprises, along an optical axis perpendicular to a specimen surface, a beam separator (e.g., an ExB), a front lens system, and a rear lens system. The beam separator is situated and configured to cause an irradiation charged-particle beam, incident to the beam separator from an off-axis source, to propagate along the optical axis to the specimen surface. The irradiation beam has an energy sufficient to cause the specimen surface to emit secondary electrons. The front lens system is situated along an axis between the beam separator and the specimen surface, and is configured to form the secondary electrons into a beam that propagates along the axis from specimen surface and forms a first intermediate image of the irradiated surface at an optical center of the beam separator. The rear lens system is situated axially between the beam separator and an imaging surface. The rear lens system comprises a relay optical system and a projection lens. The relay optical system includes multiple lenses configured to cause the beam of secondary electrons to form a second intermediate image at a second intermediate image plane located along the axis downstream of the first intermediate image plane. The projection lens is further configured to cause the beam of secondary electrons from the second intermediate image plane to propagate further downstream along the axis and form an image of the second intermediate image on the imaging surface.
By providing the rear lens system with a relay optical system (comprising multiple lenses) upstream of the projection lens, divergence of the principal ray is suppressed in the relay optical system. As a result, the second intermediate image can be formed in the relay optical system while correcting distortion aberrations. Also, zooming can be achieved by changing the axial position of the second intermediate image in a continuous manner. Desirably, the relay optical system is a full-magnification system that is substantially bi-directionally telecentric near the wide-angle end of its zoom range. This configuration is useful for further decreasing aberrations.
Yet another embodiment of a secondary-electron mapping-projection apparatus according to the invention includes a beam separator (e.g., an ExB) as summarized above. The apparatus also includes a front lens system situated along an axis between the beam separator and the specimen surface. The front lens system has a substantially bi-directionally telecentric configuration, and is further configured to form the secondary electrons into a beam that propagates along the axis from specimen surface and forms an intermediate image of the irradiated surface at an intermediate image plane at an optical center of the beam separator. The apparatus also includes a rear lens system that is situated axially between the beam separator and the detector. The rear lens system comprises a relay optical system and a projection lens. The relay optical system includes multiple lenses configured to cause the beam of secondary electrons to form a second intermediate image at a second intermediate image plane located along the axis downstream of the first intermediate image plane. The projection lens is configured to cause the beam of secondary electrons, from the second intermediate image plane to propagate further downstream along the axis and form an image of the second intermediate image on the imaging surface.
This embodiment includes characteristics of the other two apparatus embodiments summarized above. Specifically, the front lens system is substantially bi-directionally telecentric, which causes the principal ray to be incident to a first lens of the rear lens system without any divergence. Hence, aberrations (including distortions) are suppressed even in wide-field imaging. Also, the relay optical system includes at least two lenses situated upstream of the projection lens. With this configuration, an intermediate image can be formed in the relay optical system while correcting distortions. Also, zooming can be achieved by changing the axial position of the intermediate image in a continuous manner. Thus, the advantages of the bidirectional telecentricity of the front lens system are combined with the advantages of the relay optical system in the rear lens system.